Discharge lamp illumination circuit

ABSTRACT

A discharge lamp illumination circuit  1  has a DC-AC conversion circuit  3  which effects AC conversion and boosting upon receipt of a DC input, and a starter circuit  4  for supplying a start-up signal to a discharge lamp. Power output from the DC-AC conversion circuit  3  is controlled by control means  6 , thereby controlling illumination of a discharge lamp  10 . An AC transformer  7  constituting the DC-AC conversion circuit  3  is provided, and a primary circuit and a secondary circuit, both forming part of the AC transformer  7 , are insulated from each other. The start-up signal boosted by the AC transformer 7 is superposed on the AC-converted output, and the resultant signal is supplied to the discharge lamp  10 . Thus, the need for a starter transformer specifically designed for startup can be obviated.

This application claims foreign priority based on Japanese Patentapplication No. 2003-292713, filed Aug. 13, 2003, the contents of whichis incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure addresses the problem of an increase in powerloss or the like associated with an increase in the frequency of adischarge lamp illumination circuit.

BACKGROUND

One configuration of an illumination circuit of a discharge lamp (e.g.,a metal halide lamp) includes a DC power supply circuit having a DC-DCconverter, a DC-AC conversion circuit and a starter circuit. Forexample, a DC voltage output from a battery may be converted to adesired voltage in the DC power supply circuit, and is converted to anAC output in a subsequent DC-AC conversion circuit. A start-up signal (aso-called starter pulse) is superposed on the AC output, and a resultantsignal is supplied to the discharge lamp (see, e.g., Japanese patentdocument JP-A-7-142182).

However, where a voltage is converted through two stages (i.e., a DC-DCvoltage conversion and a DC-AC conversion), a circuit becomes unsuitablefor miniaturization as it becomes larger in scale. Therefore, there isemployed a configuration in which an output boosted through voltageconversion effected in a first stage of the DC-AC circuit is supplied toa discharge lamp (see, e.g., Japanese patent document JP-A-7-169583).

The foregoing configuration can suffer from a problem when the frequencyof a starter circuit of the discharge lamp is increased.

For instance, when a secondary winding of a transformer (a so-called astarter transformer) constituting the starter circuit is connected inseries to a discharge lamp, a loss in the transformer is increased whenan output frequency of a DC-AC conversion circuit is increased, which,in turn, is responsible for a decrease in efficiency. An increase in anoperating frequency (i.e., an operating frequency of a switching elementconstituting the DC-AC conversion circuit) is desirable for reducing thesize of a circuit. Minimizing power loss in the starter circuit or thelike also is desirable. A configuration in which the starter circuiteffects boosting without use of a transformer can result in drawbackssuch as a complicated circuit configuration being responsible for a costhike or hindering miniaturization.

SUMMARY

The present disclosure realtes to a compact, inexpensive discharge lampillumination circuit, wherein a DC-AC conversion circuit has thefunction of effecting AC conversion and boosting (including boosting ofa start-up signal), and a circuit configuration suitable for increasinga frequency.

To address the problems discussed above, the present disclosure providesa discharge lamp illumination circuit having a DC-AC conversion circuitwhich effects AC conversion and boosting upon receipt of a DC input, astarter circuit for supplying a start-up signal to a discharge lamp, andcontrol means for controlling illumination of the discharge lamp bymeans of controlling power output from the DC-AC conversion circuit. Thedischarge lamp illumination circuit may comprise an AC transformer(which constitutes the DC-AC conversion circuit and has a primarycircuit) and a secondary circuit that are insulated from each other. Astart-up signal boosted by the AC transformer is superposed on anAC-converted output, and a resultant signal is supplied to the dischargelamp.

The configuration of the discharge lamp illumination circuit can beprovided with the following means.

-   -   A plurality of switching elements constituting the DC-AC        conversion circuit, a resonance capacitor, and an inductance        component of an AC transformer which constitutes a resonance        circuit in combination with the resonance capacitor or an        inductance element connected to the resonance capacitor (which        specifies a drive frequency of the switching elements to a        resonance frequency or higher)    -   A custom-designed winding which is provided on the AC        transformer and to which an output terminal of the starter        circuit is to be connected    -   A rectifier element and a capacitor, which constitute the        starter circuit, and a switching element connected to the        capacitor (an output voltage produced when the switching element        is brought into conduction at the time a rise in the voltage of        the capacitor is applied to a primary winding or the        custom-designed winding of the AC transformer).

The AC conversion and boosting may be effected by use of the ACtransformer, and the AC transformer also may be utilized for boostingthe start-up signal, whereby the circuit configuration is simplified(i.e., a starter transformer specifically designed for start-up isobviated).

One or more of the following advantages may be present in someimplementations. For example, use of the starter transformer connectedin series with the discharge lamp is not required, and the efficiency ofthe starter circuit can be enhanced by diminishing a loss associatedwith an increase in the operating frequency. Thus, the present inventionis suitable for reducing the size of the circuit. Moreover, use of theAC transformer common to generation of a start-up voltage and powersupply enables simplification of the circuit configuration, which iseffective for curtailing costs.

By means of the configuration provided with the switching elements andthe resonance capacitor, the drive frequency of the switching elementsmay be specified to a resonance frequency or higher, thereby ensuringcontrol stability.

The start-up signal can be boosted by means of connecting the outputterminal of the starter circuit to the primary winding or thecustom-designed winding of the AC transformer. Moreover, a required highvoltage (i.e., a start-up voltage) is obtained by utilizing a resonancevoltage as an input to the starter circuit, which is effective forsimplifying the circuit configuration and curtailing costs.

The drive frequency of the switching elements may be set to becomehigher after illumination of the discharge lamp as compared with thedrive frequency obtained before generation of the start-up signal. As aresult, a high voltage required to start the circuit may be generatedbefore illumination of the discharge lamp, and illumination can becontrolled efficiently after illumination of the discharge lamp.

The starter circuit may be configured from a rectifier element, acapacitor, and a switching element, whereby the circuit can besimplified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a basic configuration according to thepresent invention;

FIG. 2 illustrates an example circuit illustrating a configurationpattern of a starter circuit in combination with FIGS. 3 through 6;

FIG. 3 illustrates another example of the starter circuit;

FIG. 4 illustrates an example configuration of the starter circuit usinga custom-designed winding provided on an AC transformer;

FIG. 5 illustrates an example configuration in which an input voltage issupplied from a secondary side of the AC transformer to the startercircuit; and

FIG. 6 illustrates an example configuration of the starter circuit inwhich the input voltage is supplied to the starter circuit through useof an inductance element and an auxiliary winding.

DETAILED DESCRIPTION

FIG. 1 shows an embodiment of the present invention, wherein a dischargelamp illumination circuit 1 includes a DC-AC conversion circuit 3 thatreceives power from a DC power source 2, and a starter circuit 4.

The DC-AC conversion circuit 3 is provided for effecting DC-ACconversion and boosting operation upon receipt of a voltage outputdirectly from a battery or the like. In the embodiment, the DC-ACconversion circuit 3 includes two switching elements 5H and 5L, andcontrol means 6 for driving the switching elements 5H and 5L to effectswitching control operation. Specifically, one end of the switchingelement 5H on a higher stage is connected to a power supply terminal,and the other end of the switching element 5H is grounded via theswitching element 5L on a lower stage. Further, the two switchingelements 5H and 5L are alternately activated or deactivated by thecontrol means 6. In FIG. 1, the switching elements 5H and 5L are simplydenoted by a switch symbol; however, a semiconductor switching element,such as a field-effect transistor (FET) or a bipolar transistor, may beused for the elements 5H, 5L, as shown in areas surrounded by brokenlines in FIG. 1. For instance, when FETs are used, activation ordeactivation of the FETs is specified in accordance with a drive voltagesupplied from the control means 6 to the gate. Since each of the FETshas a parasitic diode, an electric current achieved when the FETS are inan OFF state flows by way of the parasitic diodes. When bipolartransistors are used, a signal is supplied from the control means 6 tothe base of the bipolar transistors, thereby specifying activation ordeactivation of the transistors. So long as the diodes are connected inparallel to the transistors, an electric current—which is obtained whenthe bipolar transistors are in an OFF state—flows by way of the diodes.

The DC-AC conversion circuit 3 has an AC transformer 7 whose primary andsecondary circuits are insulated from each other. The present embodimentuses a circuit configuration utilizing a resonance phenomenon between aresonance capacitor 8 and an inductor or inductance component 9.Specifically, the configuration pattern may include the following.

-   -   (I) a configuration which utilizes resonance between the        resonance capacitor 8 and an inductance element;    -   (II) a configuration which utilizes resonance between the        resonance capacitor 8 and a leakage inductance of the AC        transformer 7; and    -   (III) a configuration which utilizes resonance between the        resonance capacitor 8, the inductance element, and the leakage        inductance of the AC transformer 7.

The first configuration (I) may be configured as follows. An inductanceelement 9, such as a resonance coil, is provided actively. For example,one end of the inductance element 9 is connected to the resonancecapacitor 8. The other end of the resonance capacitor 8 is connected toa node between the switching elements 5H and 5L. Further, the other endof the inductance element 9 is connected to a primary winding 7 p of theAC transformer 7.

The second configuration (II) utilizes the inductance component 9 of theAC transformer 7, thereby obviating the need to add a resonance coil orthe like. Specifically, the only requirement is to connect one end ofthe resonance capacitor 8 to the node between the switching elements 5Hand 5L, and to connect the other end of the resonance capacitor 8 to theprimary winding 7 p of the AC transformer 7.

The third configuraton (III) can utilize composite series reactanceformed from the inductance element 9 and a leakage inductance.

In any of the above configurations, a discharge lamp 10 connected to asecondary winding 7 s of the AC transformer 7 can be subjected tosinusoidal illumination on condition that the operating frequency of theswitching elements is specified to a series resonance frequency orhigher by utilizing series resonance between the resonance capacitor 8and an inductive element (i.e., an inductance component or an inductanceelement) to activate or deactivate the switching elements 5H and 5Lalternately. During drive control of the switching elements performed bythe control means 6, the elements 5H and 5L should be activatedalternately to prevent the two switching elements from being activatedsimultaneously (by way of an on-duty control). Here, provided that aseries resonance frequency is denoted as “f”; an electrostatic capacityof the resonance capacitor 8 is denoted as “Lr”; and a primary-sideinductance of the transformer 7 is denoted as “Lp1,” for instance, inthe third configuration (III), the following equation is satisfiedbefore illumination of the discharge lampf=f1=1/(2•π•{square root}{square root over (Cr•(Lr+Lp1))}),and the following equation is satisfied after illumination of thedischarge lampf=f2≈1/(2•π•{square root}{square root over (Cr•Lr)}).

The present invention can be applied without regard for theconfiguration pattern assumed by the control means 6. For instance, thefollowing configuration pattern or the like may be used. Specifically, acontrol voltage is specified by providing a circuit for controlling ano-load output voltage before illumination of the discharge lamp, or acircuit for controlling transient input power or input power in a steadystate after illumination of the discharge lamp. A pulse signal obtainedas a result of conversion of the voltage into a frequency throughV(voltage)-F(frequency) conversion is shaped, and the shaped pulsesignal is transmitted as a control signal to be delivered to theswitching elements 5H, 5L.

The starter circuit 4 supplyies a start-up signal to the discharge lamp10. An output from the starter circuit 4 on startup is boosted by the ACtransformer 7, and the boosted voltage is supplied to the discharge lamp10 (the output voltage having undergone AC conversion is superposed onthe start-up signal, and thereafter supplied to the discharge lamp).

FIGS. 2 through 6 show a configuration example of the starter circuit 4,and the following configuration patterns.

-   -   (A) A pattern in which an output terminal of the starter circuit        is connected to any point on the primary winding of the AC        conversion transformer (see FIGS. 2 and 3), and    -   (B) A pattern in which the output terminal of the starter        circuit is connected to a custom-designed winding provided on        the AC conversion transformer (see FIG. 4).

In any configuration pattern, the start-up signal can be applied to thedischarge lamp 10 from the secondary winding 7 s of the AC transformer7.

For example, as shown in FIG. 2, the pattern (A) further includes apattern in which one end of an output terminal of a starter circuit 4Ais connected to an arbitrary point on the primary winding 7 p of the ACtransformer 7, and the other end of the output terminal is connected toone end (a ground-side terminal) of the primary winding 7 p. As shown inFIG. 3, the pattern (A) also includes a pattern in which both outputterminals of a starter circuit 4B are connected to arbitrary points onthe primary winding 7 p of the AC transformer 7.

Each of the starter circuits 4A, 4B includes rectifying elements andcapacitors, and a switching element. A self-yielding element such as aspark gap, a varistor, a semiconductor element having a control terminalsuch as a thyristor, an IGBT (insulated gate bipolar transistor) or anFET can be used as the switching element. In the embodiment shown inFIGS. 2 and 3, each of the starter circuits 4A, 4B comprises twocapacitors 11, 12, diodes 13, 14, and a switching element 15.

One end of the capacitor 11 is connected to a node between the resonancecapacitor 8 and the inductance element 9 or the primary winding 7 pincluding a leakage inductance component. The other end of the capacitor11 is connected to the cathode of the diode 13 and the anode of thediode 14. The cathode of the diode 14 is connected to an arbitrary pointon the primary winding 7 p by way of the switching element 15 as well asto one end of the capacitor 12. In the embodiment shown in FIG. 2, theother end of the capacitor 12 and the anode of the diode 13 areconnected to a ground-side terminal of the primary winding 7 p.Moreover, in the embodiment shown in FIG. 3, the other end of thecapacitor 12 and the anode of the diode 13 are connected to arbitrarypoints on the primary winding 7 p. In any configuration, when theelectric charges are shifted to the capacitor 12 by means of a chargepump circuit (a voltage doubler circuit) using a diode and a capacitorand thereafter the voltage across the capacitor increases to a certainthreshold value, the switching element 15 is brought into conduction,whereupon the output voltage achieved at that time is applied to theprimary winding 7 p of the AC conversion transformer 7. The boostedstart-up signal is applied to the discharge lamp 10, whereby thedischarge lamp is illuminated.

When the number of turns of the winding between the output terminals(i.e., a portion of the primary winding 7 p) is denoted as np and thenumber of turns of the secondary winding 7 s of the AC transformer 7 isdenoted as ns, the output of the starter circuit 4 is boosted by afactor of ns/np. For instance, in the case of a discharge lamp used asthe light source for vehicle illumination, a pulse voltage of 20 kV ormore must be generated on the secondary winding. For this reason, on theassumption that the pulse voltage output from the starter circuit is onekV, a boosting ratio of 1:20 or more is required. At the time of startupof the discharge lamp, such a high voltage is required, and hence thestarter circuit has a structure in which the primary and secondarycircuits of the AC transformer 7 are insulated from each other.

For example, as shown in FIG. 4, in the pattern (B), output terminals ofa starter circuit 4C are connected to a custom-designed winding 7 awhich is smaller than the primary winding 7 p in terms of the number ofcoils, thereby boosting an output voltage. Specifically, a structuraldifference between the starter circuit 4B shown in FIG. 3 and thestarter circuit 4C is that one end of the switching element 15 isconnected to one end of the custom-designed winding 7 a, and a nodebetween the capacitor 12 and the anode of the diode 13 is connected tothe other end of the custom-designed winding 7 a. The switching element15 is brought into conduction when the voltage of the capacitor isboosted by means of shifting of the electric charges into the capacitor12, and the output voltage achieved at that time is applied to thecustom-designed winding 7 a. The number of turns “np” corresponds to thenumber of turns of the custom-designed winding 7 a, and the output ofthe starter circuit 4 c is boosted by a factor of ns/np. The start-upsignal is applied to the discharge lamp 10, thereby illuminating thedischarge lamp.

Although in each embodiment one of the input terminals of the startercircuit also serves as the output terminal, the other input terminal isconnected to a node between the resonance capacitor 8 and the inductanceelement 9 or to a node between the resonance capacitor 8 and the primarywinding 7 p of the AC transformer 7, whereby a resonance voltage can beinput to the starter circuit.

In order to generate, on the secondary winding of the AC transformer 7,a pulse voltage having a peak-to-peak value required to activate thedischarge lamp 10, the highest possible voltage must be supplied to thecapacitor provided in the starter circuit 4, to recharge the capacitor.When the recharging voltage is low, the boosting ratio should beincreased, which also involves increasing a winding ratio. Therefore,difficulty is encountered in obtaining a pulse having an idealpeak-to-peak value, or there arises a problem of an increase in ohmicloss (or rendering the winding thick to thereby reduce a power lossresults in a bulky transformer). For this reason, a high voltage can beobtained by utilization of an existing circuit element, so long as aresonance voltage formed by the resonance capacitor 8 and the leakageinductance of the AC transformer 7 or the inductance element is input tothe starter circuit 4. Thus, input of the resonance voltage is effectivefor simplifying the circuit configuration and curtailing costs.

In order to stabilize control of the discharge lamp after the dischargelamp has been illuminated by generating the start-up signal (i.e., thepulse voltage), the operating frequency of the switching elements 5H, 5Lachieved before application of the start-up signal preferably is madehigher than the operating frequency achieved after illumination of thedischarge lamp. The secondary circuit of the AC conversion transformer 7is opened before the discharge lamp is illuminated by application of thestart-up signal, whereby the transformer is deemed equivalent to a chokecoil. The series resonance frequency f1 achieved in this state is lowerthan the frequency f2 achieved during illumination. The switchingelements are controlled by means of the operating frequency in theneighborhood of f1. After illumination of the discharge lamp, theswitching elements are controlled by the operating frequency, which isin the neighborhood of the series resonance frequency f2, and determinedby the electrostatic capacitance of the resonance capacitor 8 and theleakage inductance of the AC transformer 7 or the inductance of theinductance element (or a composite inductance consisting of theseinductances).

During power control operation, switching control is preferablyperformed at an operating frequency which is higher than the seriesresonance frequency. When the operating frequency is made to coincidewith the series resonance frequency, the maximum power is extracted.Therefore, illumination of the discharge lamp can be quickly shifted toa steady state by supplying the power to the discharge lamp as initialpower to promote illumination of the discharge lamp. When switchingcontrol operation is performed with an operating frequency, which islower than the series resonance frequency, a composite impedanceconsisting of the electrostatic capacitance of the resonance capacitorand the inductance enters a capacitative region, whereby the startercircuit becomes difficult to control. Therefore, the operating frequency(switching frequency) preferably is controlled so as to avoid occurrenceof such a situation.

For instance, the operating frequency at a frequency higher than f1 iscaused to approach f1 before illumination of the discharge lamp. Afterthe discharge lamp has been illuminated by the start-up signal, it isdesirable to cause the operating frequency to approach f2 (>f1) in orderto supply power to the discharge lamp efficiently, in view ofminiaturization of the circuit scale and cost reduction.

In the configuration patterns of the starter circuit discussed above,the resonance voltage is applied to the input terminals of the startercircuit. However, application of the present invention is not limited tosuch a circuit.

For instance, as mentioned previously, a high voltage should be appliedto the input terminals of the starter circuit. When the voltage is low(i.e., when a rated voltage is low or in a situation where an actualvoltage is lower than the rated voltage), an increase in a power loss orthe rating or withstand voltage of the elements is induced by the needto increase the resonance current, thereby inducing a problem of anincrease in circuit scale or cost. When the discharge lamp isilluminated, the primary current of the AC transformer is determined bythe number of turns of the primary and secondary windings of thetransformer or the secondary current (i.e., a lamp current). The smallerthe inductance component formed by the resonance capacitor and thetransformer or the composite inductance consisting of the inductance ofthe transformer and the inductance of the inductance element, thesmaller the power loss can be made. However, if the inductance is madetoo small, there may arise a case where the resonance voltage (an inputvoltage to the starter circuit) achieved before illumination of thedischarge lamp fails to become sufficiently high. Hence, the resonancecurrent must be increased, which in turn leads to an increase in powerloss. Alternatively, when the resonance voltage is low, the number ofcircuit stages (charge pump circuits), each consisting of the capacitorand the diode provided in the starter circuit, or the boosting ratio ofthe start-up signal must be increased, which complicates the circuitconfiguration or results in a bulky transformer.

Therefore, if the low level of the voltage presents a problem, thefollowing configuration patterns may be provided.

-   -   (a) A configuration pattern in which a voltage is input to the        starter circuit from the secondary circuit of the AC conversion        transformer (see FIG. 5); and    -   (b) A configuration pattern in which there is provided an        auxiliary winding which constitutes a transformer in combination        with the inductance element connected to the resonance        capacitor, and in which a voltage is input to the starter        circuit from the auxiliary winding (see FIG. 6).

As shown in FIG. 5, in the configuration pattern (a), one of the inputterminals of a starter circuit 4D is connected to an arbitrary point onthe secondary winding 7 s of the AC transformer 7 or one end of thesecondary winding (non-ground terminal). The internal configuration ofthe starter circuit 4D shown in the embodiment is identical with that ofthe starter circuit of the embodiment shown in FIG. 2. Therefore, oneend of the capacitor 11 is connected to the secondary winding 7 s,whereby the capacitors 11, 12 are recharged. In the present embodiment,one of the output terminals of the starter circuit 4D is connected to anarbitrary point on the primary winding 7 p, and the remaining outputterminal is connected to the ground-side terminal of the primary winding7 p. However, the connection is not limited to this pattern. Both outputterminals of the starter circuit may be connected to arbitrary points onthe primary winding as shown in FIG. 3, or the output terminal may beconnected to the custom-designed winding 7 a as shown in FIG. 4. In anyevent, the maximum current value flowing through the circuit isdecreased to lessen the load on the circuit, thereby lowering the powerloss. The circuit scale can be reduced, which is advantageous tominiaturization or cost reduction.

As shown in FIG. 6, in the configuration pattern (b), the resonanceinductance element 9 is provided with an auxiliary winding 16, therebyconnecting both ends of the winding to the respective input terminals ofa starter circuit 4E. The internal configuration of the starter circuit4E described in the present embodiment is identical with that of thestarter circuit of the embodiment shown in FIG. 2. Therefore, one end ofthe capacitor 11 is connected to one end of the auxiliary winding 16,and the other end of the winding is connected to the anode of the diode13. The capacitors 11, 12 of the starter circuit 4E are recharged by thevoltage boosted through use of the inductance element 9 and theauxiliary winding 16. In the present embodiment, one of the outputterminals of the starter circuit 4E is connected to an arbitrary pointon the primary winding 7 p, and the other output terminal is connectedto the ground-side terminal of the primary winding 7 p. However, theconnection is not limited to this pattern. Both output terminals of thestarter circuit may be connected to arbitrary points on the primarywinding as shown in FIG. 3, or the output terminal may be connected tothe custom-designed winding 7 a as shown in FIG. 4. In any event, thevoltage input to the starter circuit can be freely designed by means ofsettings pertaining to the number of turns of the auxiliary winding 16.The number of components is increased by one by addition of theauxiliary winding 16. However, the need for designing a circuit inconsideration of influence on the input voltage stemming from generationof the start-up signal is obviated.

1. A discharge lamp illumination circuit comprising: a DC-AC conversioncircuit which effects AC conversion and boosting upon receipt of a DCinput, a starter circuit for supplying a start-up signal to a dischargelamp, and control means for controlling illumination of the dischargelamp by means of controlling power output from the DC-AC conversioncircuit, the discharge lamp illumination circuit comprising an ACtransformer which constitutes the DC-AC conversion circuit and has aprimary circuit and a secondary circuit, which are insulated from eachother, wherein a start-up signal boosted by said AC transformer issuperposed on an AC-converted output, and a resultant signal is suppliedto the discharge lamp.
 2. The discharge lamp illumination circuitaccording to claim 1, wherein the AC-DC conversion circuit comprises aplurality of switching elements and a resonance capacitor, and saidswitching elements are activated by the control means to effect serialresonance between said resonance capacitor and either an inductancecomponent of said AC transformer or an inductance component connected tosaid resonance capacitor.
 3. The discharge lamp illumination circuitaccording to claim 1, wherein an output terminal of said starter circuitis connected to an arbitrary point on a primary winding of said ACtransformer or a custom-designed winding provided on said AC transformeras a winding which has a smaller number of coils than does said primarywinding; and said start-up signal is applied to said discharge lamp fromsaid secondary winding of said AC conversion transformer.
 4. Thedischarge lamp illumination circuit according to claim 2 wherein aninput terminal of said starter circuit is electrically connected to anode between said resonance capacitor and said AC conversion transformeror to a node between said AC conversion transformer and said inductanceelement, thereby inputting a resonance voltage of said DC-AC conversioncircuit to said starter circuit.
 5. The discharge lamp illuminationcircuit according to claim 2 wherein a drive frequency of said switchingelement obtained before generation of said start-up signal is higherthan a drive frequency of said switching element obtained aftergeneration of said discharge lamp.
 6. The discharge lamp illuminationcircuit according to claim 1 wherein said starter circuit comprises arectifier element, a capacitor, and a switching element connected tosaid capacitor; and wherein an output voltage produced when saidswitching element is brought into conduction at the time of boosting ofthe voltage of said capacitor is applied to a primary winding or saidcustom-designed winding of said AC transformer.